Chair base



FIG. 1 is a perspective view of chair base showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left elevational view thereof;

FIG. 5 is a right elevational view thereof;

FIG. 6 is top plan view thereof; and,

FIG. 7 is bottom plan view thereof.

The ornamental design for chair base, as shown and described.